1. Field of the Invention
The present invention relates to a gain controlled amplifier circuit, and more specifically, to a gain controlled amplifier circuit suitably employed, for example, in a surround-decoder system for an audio signal processing of a large sized television, and suitable for an IC (Integrated Circuit).
2. Description of the Background Art
An article related to a Dolby Pro Logic Surround System proposed by Dolby Laboratories Licensing Corp. of the United States appears on pp. 88-89 in "NIKKEI Electronics" issued on June 27, 1988. As shown in FIG. 2, the system is comprised of left and right input terminals 1 and 2 to which left and right stereo signals L.sub.T and R.sub.T are applied, a first adder circuit 3 for adding the left and right stereo signals L.sub.T and R.sub.T to generate a sum signal C' (=L.sub.T +R.sub.T), a subtractor circuit 4 for subtracting the right stereo signal R.sub.T from the left stereo signal L.sub.T to generate a difference signal S' (=L.sub.T -R.sub.T), first and second detection circuits 5 and 6 for respectively detecting levels of the left and right stereo signals L.sub.T and R.sub.T, third and fourth detection circuits 7 and 8 for respectively detecting levels of the sum and difference signals C' and S', a first level ratio detection circuit 9 for detecting a level ratio of the output signals of the first and second detection circuit 5 and 6, a second a level ratio detection circuit 10 for detecting level ratio of the output signals of the third and fourth detection circuits 7 and 8, a VCA (Voltage Controlled Amplifier) 11 including a plurality of gain controlled amplifier circuits (not shown) each for controlling the level of the left or right stereo signal L.sub.T or R.sub.T in response to any one of the output signals of the first and second level ratio detection circuits 9 and 10, a second adder circuit 12 for selectively adding the left and right stereo signals and output signals of the VCA 11 to generate left and right stereo output signals L and R, a center output signal C and a surround output signal S, a center mode control circuit 13 for switching between the left and right stereo output signals L and R and the center output signal C in response to a mode, and a passive decoder 14 for performing signal processing such as delay and noise reduction to the surround output signal S. Therefore, a structure such as shown in FIG. 2 enables signal processing of enhancement of direction to clarify surround localization of sound, so that acoustics having presence can be provided to listeners. Particularly, the system is effective when applied to an audio signal processing of a large-sized television, so that it can produce the same effect on an audience as that obtained when seeing a movie at a theater.
For the gain controlled amplifier in the VCA 11 of FIG. 2, a differential gain controlled amplifier circuit as shown in FIG. 3 can be employed. In FIG. 3, a gain controlling signal Vc is applied between bases of differentially connected first and second transistors 15 and 16, and an input signal to be amplified such as L.sub.T or R.sub.T is applied to a base of a current source transistor 17 having its collector connected to a common emitter of the first and second transistors 15 and 16. Then, an output signal is provided from a collector of the second transistor 16 to an output terminal 18. In FIG. 3, when collector currents of the first and second transistors 15 and 16 are represented as i.sub.1 and i.sub.2, respectively, the relation between the controlling voltage Vc applied between control terminals 19 and 20 and the collector currents i.sub.1 and i.sub.2 will be represented in the following equation: ##EQU1## Wherein
q: electric charge of electron
k: Boltzmann's constant
T: absolute temperature
In addition, if a collector current of the current source transistor 17 flowing in response to a level of an input signal is represented as i.sub.IN, the following relation will be established. EQU i.sub.IN =i.sub.1 +i.sub.2 ( 2)
Accordingly, if equations (1) and (2) are applied, an output current i.sub.OUT obtained at the output terminal 18 will be represented in the following equation: ##EQU2## and, a current gain G.sub.i of the circuit shown in FIG. 3 will be represented in the following equation: ##EQU3##
As apparent from equation (4), the circuit of FIG. 3 has a problem in linearity of its amplification characteristics.
Especially when used in such a surround-decoder system as shown in FIG. 2, if linearity of VCA 11 is bad, a balance of the left and right stereo output signals L and R, the center output signal C and the surround output signal S is affected, so that enough surround effect could not be obtained.